1. Field of the Invention
This invention relates to a method and apparatus for the rewarming of hypothermic humans.
2. Description of the Prior Art
In the profoundly hypothermic human, the lowered ventilation rate causes less heat donation by the inhalation method and provides "stabilization" of core temperature during transport to hospital. In mild or moderate hypothermia, actual "rewarming" of the victim is desirable. In such victims, ventilation rate is elevated and a satisfactory rewarming rate of the critical core is obtained. Victims who are hypothermic and lack vital signs (e.g., cold water near-drowning) should not be rewarmed until arrival at the hospital. Inhalation therapy based on the spontaneous ventilation of the victim ensures that this will be the case.
Accidental hypothermia is a common hazard to man's endeavours in cold air and water environments. Because of the potential for rapid heat loss in cold water immersion and in mountain accidents, accidental hypothermia often becomes a true medical emergency with substantial mortality. Yet, despite the long-standing recognition of this problem, a single definitive plan for first-aid therapy of accidental hypothermia has not been accepted. Because hypothermia fatalities occasionally occur even after removal of the victim from his cold environment, organizations involved in the recovery of hypothermia victims should have the capacity to give effective emergency medical treatment.
A traditional way of treatment of accidental hypothermia has been rapid peripheral rewarming. This has been effective even in the profoundly hypothermic victim. However, the peripheral vasodilation which usually accompanies most forms of external rewarming may precipitate the well-described "afterdrop" in core body temperature, which can potentiate the possibility of ventricular fibrillation from further cooling of the myocardium. The peripheral vasodilation accompanying rapid external rewarming may also produce "rewarming shock", a form of hypovolemia secondary to diminished central blood volume. This latter effect is particularly evident in hypothermia of slow onset, where intravascular volume is decreased secondary to fluid shifts. Hence, rapid peripheral rewarming of the accidental hypothermia victim is attended by a number of potential hazards which cannot be well managed in the non-hospital setting.
It is now established that the most effective and safe method for rewarming of severely hypothermic humans is by donation of heat directly to the core of the body rather than via the periphery. Accordingly, hospital management of such victims most frequently involves peritoneal dialysis (warm saline in the abdominal cavity) and inhalation (or airway) rewarming. Of these two methods, only inhalation rewarming is suitable for use by non-medical personnel at the rescue site or during transport to hospital. Inhalation of warm (about 42.degree.-45.degree. C.), water-saturated air or oxygen donates heat directly to the head, neck and thoracic core (the critical core). Although the amount of heat donated by this method is not large, its transfer to the most important tissues of the body is strategic, and results in minimum "afterdrop" of core temperature and does not stimulate return of peripheral blood with possible elevations of acidity and potassium concentration.
Methods of inhalation rewarming are now used routinely in hospitals and an electrically powered inhalation rewarming unit has been developed for the field. However, electricity is not always available in remote regions where hypothermic victims are found. Also, such units have the disadvantage of requiring a source of compressed gas (e.g., oxygen).
Some reports of inhalation rewarming techniques are by M. L. Collis et al "Accidental Hypothermia: An Experimental Study of Practical Rewarming Methods" Aviation, Space, And Environment Medicine, July 1977, p. 625; and by E. L. L. Lloyd "Accidental Hypothermia Treated by Central Rewarming Through the Airway", Brit. J. Anaeth. (1973) 45, p. 41.
The patent literature is replete with devices that pertain to warming and humidifying air or anaesthesia gases. Among these patents are the following U.S. Pat. Nos.:
______________________________________ 3,434,471 3,506,003 3,526,222 3,638,926 3,659,604 3,820,540 3,871,373 3,902,883 3,974,830 3,912,795 3,771,721 Re. 30046 3,664,337 3,630,196 3,598,116 3,326,214 3,565,072 3,902,488 4,028,445 4,038,980 4,051,205 4,060,576 4,121,583 4,016,878 4,019,511 4,026,285 4,007,238 4,009,713 4,084,587 4,101,611 ______________________________________